If we have, f,e, , an example: a room can not exist without a building, but one building can have 0 to many rooms, does it mean that a room as a weak E cannot have key attribute?Can U give me an example?Many thanks!

I'm not sure if this is a strong example, but I think this is considered a Weak Entity..

Consider an order system. In many systems you have an Order (Header) and an Order Detail (or Line Item). The Order Detail doesn't make any sense without the Order itself. So the Order Detail (or Line Item) would be considered a Weak Entity becuase it needs the Order (Header) to (FK to Order) exist in order for you to have an Order Detail (Line Item) row.

Honestly, I wouldn't get hung up in classifications of entities. Just know there are Enties, Attributes and Relationships. Master that, and you'll do fine.

I agree with Lamprey; it is all relative depending on how you design your data model.For an attribute to be a key, it should be uniquely identifiable.Here is another example, if you want to create a table containing information about your friends. You may create a table like this:

Since two friends have the same name, and two of them have the same birthday you cannot use either name or birthday as keys. Since Phil does not have a Cellphone you cannot use CellPhone as a key either. In this case you can do couple of things to create a key; You can combine Name, birthday and Cellphone together as a key, because it is unlikely that two friends will have the same name, birthday and cellphone number; together they should form a unique identifier. Or you may choose to add a new attribute to the table called ID and assign a unique number for each of your friends.